判断数据类型

typeof

• 可识别标准类型（null会识别为object）
• 不能识别具体的对象类型（Function除外）

console.log(typeof "jerry");//"string"
console.log(typeof 12);//"number"
console.log(typeof true);//"boolean"
console.log(typeof undefined);//"undefined"
console.log(typeof null);//"object"
console.log(typeof {name: "jerry"});//"object"

console.log(typeof function(){});//"function"
console.log(typeof []);//"object"
console.log(typeof new Date);//"object"
console.log(typeof /\d/);//"object"
function Person(){};
console.log(typeof new Person);//"object"

识别null的方法是进行恒等比较

console.log(typeof null);//'object'
console.log(null === null);//true
console.log(undefined === null);//false
console.log('null' === null);//false

instanceof

• 不能识别标准类型
• 可以识别具体的对象类型
• 不能识别null和undefined

console.log("jerry" instanceof String);//false
console.log(12 instanceof Number);//false
console.log(true instanceof Boolean);//false
console.log(undefined instanceof Undefined);//报错
console.log(null instanceof Null);//报错
console.log({name: "jerry"} instanceof Object);//true

console.log(function(){} instanceof Function);//true
console.log([] instanceof Array);//true
console.log(new Date instanceof Date);//true
console.log(/\d/ instanceof RegExp);//true
function Person(){};
console.log(new Person instanceof Person);//true
console.log(new Person instanceof Object);//true

instanceof是一个二元运算符，左操作数是一个对象，右操作数是一个构造函数。如果左侧的对象是右侧构造函数的实例对象，则表达式返回true；否则返回false
如果左操作数不是对象，返回false，如果右操作数不是函数，则抛出一个TypeError

console.log(123 instanceof function(){});//false
//Uncaught TypeError: Right-hand side of 'instanceof' is not an object
console.log({} instanceof 123);

constructor

• 可以识别标准类型和具体的对象类型
• 不能识别null和undefined
  实例对象的constructor属性指向其构造函数
  如果是内置类型，则输出function 数据类型(){[native code]}；如果是自定义类型，则输出function 数据类型(){}

console.log(("jerry").constructor);//function String(){[native code]}
console.log((12).constructor);//function Number(){[native code]}
console.log((true).constructor);//function Boolean(){[native code]}
console.log((undefined).constructor);//报错
console.log((null).constructor);//报错
console.log(({name: "jerry"}).constructor);//function Object(){[native code]}

console.log((function(){}).constructor);//function Function(){[native code]}
console.log(([]).constructor);//function Array(){[native code]}
console.log((new Date).constructor);//function Date(){[native code]}
console.log((/\d/).constructor);//function RegExp(){[native code]}
function Person(){};
console.log((new Person).constructor);//function Person(){}

Object.prototype.toString()

• 可以识别标准类型和内置对象类型
• 不能识别自定义对象类型
  对象的类属性是一个字符串，用以表示对象的类型信息。javascript没有提供设置这个属性的方法，但有一种间接方法可以查询它。Object.prototype.toString()方法返回了如下格式的字符串：[object 数据类型]

console.log(Object.prototype.toString.call("jerry"));//[object String]
console.log(Object.prototype.toString.call(12));//[object Number]
console.log(Object.prototype.toString.call(true));//[object Boolean]
console.log(Object.prototype.toString.call(undefined));//[object Undefined]
console.log(Object.prototype.toString.call(null));//[object Null]
console.log(Object.prototype.toString.call({name: "jerry"}));//[object Object]

console.log(Object.prototype.toString.call(function(){}));//[object Function]
console.log(Object.prototype.toString.call([]));//[object Array]
console.log(Object.prototype.toString.call(new Date));//[object Date]
console.log(Object.prototype.toString.call(/\d/));//[object RegExp]
function Person(){};
console.log(Object.prototype.toString.call(new Person));//[object Object]